javascript code to generate calendar. By using this you can create your own customized calendar UI.

calendarDates.js

const daysInMonth = (iYear, iMonth) => 32 - new Date(iYear, iMonth, 32).getDate()

const months = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']

const getSelectedMonthDates = (selectedYear, selectedMonth) => {
  const calendarDates = []
  const firstDay = (new Date(selectedYear, selectedMonth)).getDay() // 0 = 'Sunday', 6 = 'Saturday'
  const days = daysInMonth(selectedYear, selectedMonth) // how many days in a month

  let x = 0
  for (let i = 0; i < days; i += 1) {
    if (calendarDates.length === 0) {
      calendarDates.push([])

      if (firstDay !== 0) {
        for (let y = 0; y < firstDay; y += 1) {
          const getYear = months[selectedMonth] === 'January' ? selectedYear - 1 : selectedYear
          const getMonth = months[selectedMonth] === 'January' ? 11 : selectedMonth - 1
          const previousMonthDays = daysInMonth(getYear, getMonth)
          calendarDates[x].unshift({ date: previousMonthDays - y, month: getMonth, year: getYear })
        }

        calendarDates[x].push({ date: i + 1, month: selectedMonth, year: selectedYear })
      } else {
        calendarDates[x].push({ date: i + 1, month: selectedMonth, year: selectedYear })
      }
    } else if (calendarDates.length !== 0) {
      if (calendarDates[x].length < 7) {
        calendarDates[x].push({ date: i + 1, month: selectedMonth, year: selectedYear })
      } else {
        calendarDates.push([{ date: i + 1, month: selectedMonth, year: selectedYear }])
        x += 1
      }
    }
  }

  const lastIndex = calendarDates.length - 1

  if (calendarDates[lastIndex].length < 7) {
    const leftOverDay = 7 - calendarDates[lastIndex].length
    for (let i = 0; i < leftOverDay; i += 1) {
      const getMonth = months[selectedMonth] === 'December' ? 0 : selectedMonth + 1
      const getYear = months[selectedMonth] === 'December' ? selectedYear + 1 : selectedYear
      calendarDates[lastIndex].push({ date: i + 1, month: getMonth, year: getYear })
    }
  }

  return calendarDates
}

const presentDate = new Date()
const presentMonth = presentDate.getMonth() // 0 - 11, 0 = 'January'
const presentYear = presentDate.getFullYear()

const dates = getSelectedMonthDates(presentYear, presentMonth)
console.log(dates)
/*
  Expected output 2D array
  The nested array length will always be 7, index 0 = 'Sunday', index 6 = 'Saturday
  e.g.
  [
    [{ month: 1, day: 31 }, { month: 2, day: 1 }, { month: 2, day: 2 }, { month: 2, day: 3 }, { month: 2, day: 4 }, { month: 2, day: 5 }, { month: 2, day: 6 }],
    ...
  ]
*/